Orson Scott Card never ceases to amaze me. I read this after Ender's Game, which I though was the height of Card's writing, and found I was wrong. This detailed, somewhat historical novel is one of my all-time favorites.
Future people have the ability to watch history though a certain machine they have. Eventually they realize they can also change the past - and they decide that if only Christopher Columbus hadn't reached the Western World, life would be much better today...
I can't recommend this highly enough!